This invention relates to waveguides, and more particularly to an apparatus for interfacing a first diameter waveguide to a second diameter waveguide, where the second diameter waveguide has a larger diameter than the first, and without causing reflections of a wave propagating through the apparatus from the first to the second waveguides.
In a microwave communication system, microwave signals generally must travel through many different types of transmission lines and waveguide structures as they propagate between an input and an output port. All of these different types of propagating media have advantages and disadvantages in and of themselves and in their ability to interface to other parts of a microwave communication system. The other parts of the microwave communication system may include antennas, printed microwave circuitry, and machined waveguide circuitry.
A specific problem that arises in transmitting microwave signals is when the microwave signal must propagate from a small diameter waveguide to a large diameter waveguide. One specific application where this problem arises is in connection with the testing of presently designed phased array antenna modules. One such phased array antenna module is manufactured by The Boeing Co. With this antenna module, each radiating element of the module is constructed as a circular, dielectric loaded waveguide having a diameter of approximately 0.236 inch (6 mm). However, test equipment required to interface to the circular waveguide of each radiating element requires a circular waveguide having a diameter of approximately 0.590 inch (14.97 mm). A microwave transition is therefore required to transfer electromagnetic energy to and from the waveguide used to interface with the test equipment.
A general restraint on this design is the need for minimum length to separate the two waveguides. It is preferred that this transition length be as short as possible to facilitate ease of test setup. One particular method of solving this problem would be to construct a long taper between the loaded small diameter waveguide and the larger unloaded waveguide. This is illustrated in FIG. 1. However, this method is presently not possible using current available dielectric material because of the brittle nature of such presently available material. The brittle nature of such material prevents the needed cone shape with a sharp tip from being machinable. Because of this, the tapered approach to solving the above-described problem has not been pursued.
Even if suitable dielectric material was available which allowed manufacturing of a cone shaped portion of dielectric with a very sharp tip, other problems would likely have been encountered in implementing this solution. The first problem would be to maintain the correct cutoff frequency of the dominant mode TE11 versus the taper angle. If the taper angle is too severe verses the increasing diameter (i.e., 0.236 inch to 0.590 inch), then at some point along the length of the transition the cutoff frequency would be above the operational frequency. Thus, the signal would not be able to propagate past this point. Even if a successful taper angle could be found, the length of the dielectric would produce a transition distance that would be too long to be practical for use in present day test environments.
A second design approach which has been considered is a series of quarter wave steps. One such quarter wave step is shown in FIG. 2. This approach, however, is only successful when the step transitions do not have a large reactive component. In a transition with large steps such as that shown in FIG. 2, the reactance is large. This produces an impedance mismatch resulting in reflection of the propagating wave through the step portion.
Accordingly, some form of apparatus is needed which acts as a transition between two waveguides, where one waveguide has a smaller diameter than the other, and which does not cause any reflections of the propagating wave as it propagates through the apparatus.
It would further be highly desirable to provide an apparatus which forms a transition between a first waveguide and a second waveguide, where the second waveguide has a larger diameter than the first waveguide, and where a propagating wave is able to pass through the apparatus without being reflected, and further where the apparatus forms a very small distance which makes it suitable for use in test environments as an interface between two such waveguides of different diameter.
The present invention is directed to an apparatus for forming a transition between a waveguide having a first diameter and a waveguide having a second diameter, where the second diameter is larger than the first diameter, and which allows a microwave signal to propagate therethrough without causing reflections of the propagating wave. The apparatus further accomplishes this with a short overall length which makes the apparatus suitable for use with present day test equipment being used to test present day phased array antennas, where the phased array antenna includes a plurality of radiating elements each having a circular waveguide associated therewith.
The apparatus of the present invention forms a quarter wave step but without producing the reactive components typically associated with quarter wave step matching devices. The quarter wave step of the present invention is formed, in one preferred form, from a section of material which has a first diameter bore formed therethrough. A second bore is formed concentrically with the first bore and has a diameter larger than the first bore, and sufficient to form the quarter wave step. A third bore is formed in the apparatus concentrically with the first and second bores. The third bore has a third diameter. The first diameter matches the diameter of the circular waveguide of the radiating element being tested. The third diameter matches the diameter of the waveguide associated with the test equipment. A first ring resonator is disposed within the second diameter adjacent the point where the first diameter transitions into the second diameter. A second ring resonator is disposed at the point of transition between the second diameter and the third diameter. In a preferred embodiment, each of the ring resonators are formed on separate circular printed circuit boards which are placed within the apparatus. The ring resonators cancel the reactive component introduced by the quarter wave step which prevents reflections of a propagating wave being caused as it passes through the quarter wave step.
In a preferred embodiment of the present invention the apparatus is formed from a single block of metallic material such as aluminum. The first bore forms a first waveguide and is preferably filled with a dielectric material. The second bore forms the quarter wave step and is preferably filled with a dielectric material with a different dielectric constant than the first waveguide. The third diameter forms a second waveguide and is also air filled. Advantageously, the apparatus is relatively short in length and therefore well suited for use in a test environment when interfacing a waveguide associated with a radiating or receiving element of a phased array antenna with test equipment.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.